The present invention relates to a thermocouple probe and, more specifically, to a probe for a thermocouple for accurately sensing high temperatures.
Some common physical properties which change with temperature are the length of a rod, the volume of a liquid, the electrical resistance of a wire, and the color of a lamp filament. All of these changes are utilized in the construction of various types of thermometers. Commonly used thermometers include the liquid-in-glass thermometer, the thermister, the optical pyrometer and the constant volume gas thermometer. Another commonly used thermometer is the thermocouple.
The thermocouple consists of an electrical circuit in which wires of any two unlike metals are joined at both ends to form a complete circuit. Whenever the two junctions of the wires are at different temperatures, an electromotive force exists in the circuit. In a typical application, one junction is maintained at a known, reference temperature, the other junction is placed in the region of the unknown temperature and the EMF is indicated on a suitable measuring device, such as a galvanometer. The junction which senses the unknown temperature may be formed into a thermocouple probe and is referred to commonly as the hot junction because thermocouples are usually used for measuring temperatures higher than room temperature.
In many areas of research and experimentation it is necessary to measure accurately high temperatures. In a typical area, the surface area of certain materials are analyzed at temperatures which may range from 400.degree. to 1600.degree.C. Such analysis is keyed on the ability to measure accurately the temperatures in this range. Typically, this range of temperature is generated by a suitable oven which may operate under reducing atmosphere conditions. The temperature probe must be inserted into the oven together with the sample to be analyzed. Commonly, thermister devices are used for sensing temperatures at this range. For example, a platinum resistance thermometer may sense temperatures up to 1760.degree.C, the melting point of platinum. However, for reasons of economy, ease of use and optimum use of space for sensing temperatures, it is desirable to use the thermocouple for these temperature ranges.
It was found, however, that no thermocouple probes could be found which would accurately measure temperatures in the range of 400.degree. to 1600.degree.C. One thermocouple probe which was tried unsuccessfully included a chromel-alumel thermocouple, a double bore ceramic insulator, a stainless steel sheath and fiber glass sleeving at the cold end. The sleeving comprised a series of individual sleeves of short length stacked adjacent to one another around the length of the conductors extending from the cold end of the thermocouple to the electrical connector. Besides not accurately sensing the temperatures in this range, the available thermocouples of which the inventor is aware have other features which make them unsuitable for high temperature measuring. First, the probes do not have suitable means for being mounted in the oven. Also, the hot junction protrudes outwardly from the lower end of the probe and is therefor subject to being damaged. Lastly, no suitable means for handling the probe has been found. The probe jacket is much too hot to be picked up manually, and lifting the probe by the conductors subjects the conductors to undesirable stress.